I had this problem the other day. New server. I have one of the most geared pve merc healers on the server. I was taking my little lvl 50 alt on a BT HM run. I've done it a million times so no, I didn't think we really needed to CC every trash mob.
A sorc CCed three trash mobs that I wasn't expecting. I used an AoE and marvelously broke all three. Ok, yeah, not the best move on my part but it's trash. The conversation went like this:
Sorc: Wow, how did you manage to break all three CCs?
Me: lol I wasn't expecting you to CC the trash.
Sorc: BUT ALL THREE?
Me: I used an AoE....
Sorc: Well you need to knock it the **** off. I'm getting sick of it.
He then made some jerk remarks about our healer (from my guild) not healing full time and that we probably couldn't do the optional boss. We never wiped once except this d-bag died on the trash. He acted rude the rest of the instance.
Personally, I don't think you should pug stuff if you aren't going to be a little more lenient with people (and aren't in a voice chat of some kind together). Communication mishaps happen. Just chill out, gamers. If you aren't with your hardcore gaming group (and even if you are), mistakes happen. No one is going to want to game with you if you bad mouth them or act rude the entire time. Needless to say, I will never be bringing this guy on any instances in the future.